884 Dr. Makower and Mr. Evans : Deflexion by Magnetic 



The method of finding Up just described is, however, open 

 to certain objections, for it is necessary to make two separate 

 experiments with a direct and reversed magnetic field in order 

 to obtain the deflexion suffered by the recoil-streams. A 

 further disadvantage is that a somewhat large aperture of 

 width, 0*5 mm., was used. Some other experiments were 

 therefore made as follows with a narrower slit and finer wire. 

 The apparatus used is shown in fig. 3. 



Ffc. 3. 



An active wire W of diameter 0*3 mm. was mounted as 

 shown in the figure, 1*25 cm. from the slit S which was 

 3*6 cm. from the strip C placed to receive the recoil-atoms. 

 The whole apparatus was enclosed as before in a glass 

 vessel which could be quickly evacuated and placed between 

 the poles of the electromagnet. The wire W was fixed in 

 position, the field applied and the glass vessel quickly 

 evacuated, and the radium B recoiling from the radium A 

 on the wire allowed to fall on the plate for three minutes. 

 The field was then reversed and the recoil allowed to 

 proceed for another seven minutes until the radium A on 

 the wire had decayed to an inappreciable quantity. The 

 strip C was then removed and placed on a photographic 

 plate in the dark. The radium B distributed over the strip 

 would itself have little or no effect on the photographic plate, 

 but as it decayed radium C was produced in situ, and this by 

 reason of the rays given out by it made an impression on the 

 plate which could be developed in the ordinary way. The 

 result of this experiment is shown in fig. 4 (PI. XVIII.). The 

 two bands on the plate are due to the radium B reaching the 

 plate with the direct and reversed magnetic fields respectively. 

 It will be noticed that the bands are of considerable width and 

 their edges not sharp, indicating that the particles of radium 

 B on reaching the metallic strip are scattered before being 

 stopped. That this should be so was to be expected con- 

 sidering the relatively low velocity with which the particle 

 must travel. In spite of this scattering, however, it is 

 possible to measure the distance between the middles of the 

 two bands with reasonable accuracy by the method of pro- 

 jecting an image of the photograph on to a screen by means 

 of a lantern and measuring the magnification thus produced 



